The present invention relates to a method of preparing a (Bi,Pb)SrCaCuO-2223 superconductor, and in particular a (Bi,Pb)SrCaCuO-2223 superconducting wire. The superconductor with a nominal composition of (Bi,Pb).sub.2 Sr2Ca.sub.2 Cu.sub.3 O.sub.x, the 2223 phase with a superconducting transition temperature of 110 K, has a variety of industrial applications. Brides its uses in the monolithic forms, it has been made into superconducting composite wires with promising performance for an even more wide range of engineering applications. A successful method of preparing a superconducting wire is the so-called oxide powder-in-tube (OPIT) process (S. Jin et al., U.S. Pat. No. 4,952,554, 1990). The OPIT process includes the three stages of preparing a powder of superconductor precursor oxides (precursor powder preparation stage); filling the precursor powder into a metal tube and reducing the cross section of the tube through mechanical deformation such as swaging, rolling or drawing, and for multifilamentary articles, assembling the previously formed bundles and further deforming the assembly into a multifilamentary wire or tape (mechanical deformation stage); and subjecting the composite to a sintering process, and if necessary, repeated deformation (pressing or rolling) and sintering processes, to obtain the desired superconducting properties (thermomechanical processing stage). The precursor powder preparation stage provides the starting precursor powder with appropriate composition, phase assembly, and particle size. The mechanical deformation stage provides the required geometry of the composite and more importantly the texture formation of the precursor powder. The thermomechanical processing stage is responsible for the final phase transformation into the superconducting 2223 phase and related superconducting properties. Since the 2223 phase is thermodynamically metastable and will decompose at the sintering temperature, direct use of the 2223 phase as the starting powder for the OPIT process will result in poor superconducting properties. Therefore, a powder of precursor oxides corresponding to the cation composition of the final 2223 phase is used as the starting powder. The 2223 phase has a wide solid solution range and the actual composition is slightly different from the nominal composition of (Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x. For example, a commonly used composition is Bi.sub.1.72 Pb.sub.0.34 Sr.sub.1.83 Ca.sub.1.97 Cu.sub.3.13 O.sub.x. The precursor powder, upon one or more subsequent chemical reactions, is then converted into the 2223 superconducting material in combination with greater or lesser amounts of secondary phases. Because the desired 2223 superconducting material is formed by a series of chemical reactions, the superconducting properties will depend on the chemical and phase composition of the starting materials and on the subsequent processing conditions, such as temperature, time, and oxygen partial pressure.
A common phase composition of the precursor powder consists of a tetragonal or orthorhombic 2212 phase, and one or more nonsuperconducting phases necessary for the final conversion into the 2223 phase. The tetragonal 2212 phase has equivalent a and b axes with a lattice parameter of about 5.4 angstroms. The conversion of the tetragonal to the orthorhombic phase corresponds to the formation of an oxygen deficient structure with unequal a and b axes, as described in R. Flukiger et al., Phase formation and critical current density in Bi,Pb(2223) tapes, Supercond. Sci. Technol. 10 (1997) pages A68-A92. The conversion occurs simultaneously with the incorporation of a dopant having a variable oxidation state, i.e., Pb or Sb, into the structure. The secondary phases may be considered desirable secondary phases such as, (Ca,Sr).sub.2 CuO.sub.3, CuO, (Ca.sub.2-x Srx)PbO.sub.4 and (Ca.sub.2-x-y Sr.sub.x Cu.sub.y)(Pb.sub.1-n Bi.sub.n) O.sub.z (3221 phase), which promote the formation of the 2223 phase and decrease the size of impurity phases, depending on the sintering conditions. Undesirable second phases may include 2201, 3221, CaCuO.sub.2, and un-reacted metal oxides depending on the particular sintering conditions.
When tetragonal 2212 is used in the precursor powder, a transient liquid phase is first formed and tetragonal 2212 is converted into Pb-doped orthorhombic 2212 during the sintering process. The Pb-doped 2212 phase then reacts with other oxide phases to form the 2223 phase. The transient liquid will promote densification and grain growth. However, oxygen must be released during the conversion from tetragonal 2212 to orthorhombic 2212 phase. The oxygen release may be limited by the diffusion of oxygen through the silver sheath and newly formed phases, and inhomogeneous phase formation may occur due to the difference in local oxygen partial pressure, and bubbles may form in the composite wire due to the released oxygen. Repeated pressing/rolling and sintering are often necessary to achieve a high critical current density. A prior art method releases the oxygen before the sintering process by adding a tetragonal to orthorhombic conversion process in the mechanical deformation stage (Q. Li, et al., U.S. Pat. No. 6,069,116, 2000). Although this approach is successful in achieving a high critical current density, the diffusion of oxygen through the silver sheath is a time consuming process. Moreover, the choice of sheath materials is limited to silver-based materials as silver is the only metal permeable to oxygen. When the 2212 phase in the precursor powder is a fully Pb-doped orthorhombic phase, all the Pb in the final composition is essentially in the 2212 phase. Faster reaction kinetics and more complete transformation into the 2223 phase were observed for such a precursor powder, as discussed in S. E. Dorris, et al., Methods of introducing lead into bismuth-2223 and their effects on phase development and superconducting properties, Physica C 223 (1994) pages 163-172. Furthermore, the orthorhombic 2212 phase seems to develop better deformation texture during the mechanical deformation process. However, the connectivity may be not as good compared with the powder with the tetragonal 2212 phase and the critical current density may be adversely affected accordingly.
It is recognized the transformation from tetragonal to orthorhombic 2212 is a continuous process depending on the temperature, oxygen partial pressure, and time of the calcination process. The Pb content and the lattice parameters of the 2212 phase are between the two extreme cases. A well-controlled calcination process should produce the desired 2212 phase with a narrow Pb content range. However, variations in the processing condition can produce a powder with Pb content of the 2212 phase spreading the whole composition range. The temperature and oxygen partial pressure range of each phase can be obtained from published phase diagrams. The powder phase composition moves from tetragonal 2212 to orthorhombic 2212 with increasing Pb content as the temperature or oxygen partial pressure shifts from the tetragonal range to the orthorhombic range. It seems that the best results in critical current density have been obtained from precursor powders with the tetragonal 2212 phase and better reproducibility is obtained from precursor powders with fully doped orthorhombic 2212 phase.
It is also suggested that presence of certain amount of the 2223 phase in the precursor powder may act as seeds to promote the 2223 formation kinetics and improve the critical current density (K. Sato et al., U.S. Pat. No. 5610123, 1997). However, the values of critical current density obtained by this method are not as high as the best results from the precursor powders with the tetragonal 2212 phase. The precursor powder preparation methods can also be divided into two categories according to the calcinations method: so-called one-powder process and two-powder process (see, for example, J. Jiang and J. S. Abell, Effects of precursor powder calcination on critical current density and microstructure of Bi-2223/Ag tapes, Supercond. Sci. Technol. 10 (1997) pages 678-685). In a one-powder process, all the materials are mixed and calcined together. In a two-powder process, the preparation of the Pb-doped 2212 phase is separated from the preparation of the remaining oxide phases, then the two powders are mixed to form the precursor powder (S. E. Dorris et al., U.S. Pat. No. 5,468,566, 1995, and S. E. Dorris, et al., U.S. Pat. No. 5,354,535, 1994). The two-powder process may offer better quality control since the phases and particle sizes of the two powders can be controlled independently.
As for the sheath material, a silver-based material is the material of choice. Noble metals are the only metallic materials that do not have adverse reactions with the superconducting precursor powder. Among the noble metals silver is the only material permeable to oxygen. Silver-based materials include silver, silver alloys with other noble metals, and silver enhanced with dispersed oxides such as MgO. Direct use of other metals as the sheath material is not feasible due to chemical reaction and oxygen diffusion. For short samples of a few centimeters in length, oxygen can pass through the ends of the wire, but this is not practical for industrial wires of hundreds of meters in length. A proposed prior art method to reduce the material cost is to use an inexpensive metal to replace silver at the outer surface and there are holes in the metal packed with silver so that oxygen diffusion can take place at these openings (S. Hagino, et al., U.S. Pat. No. 4,983,576, 1991, and S. Hagino, et al., U.S. Pat. No. 5,068,219, 1991). However, such a construction makes the mechanical deformation and the thermomechanical processing very difficult. It is also well known that there is a dense, well-textured layer of about 2-3 .mu.m in the 2223 phase at the silver interface, which has a very high critical current density. At the center of a superconducting filament, the microstructure may be porous, the texture may be not as good, and the critical current density may be lower. A very high critical current density value is obtained in a prior art wire-in-tube method where a thin layer of superconductor is formed between the gap of the inside silver wire and the outside silver tube (U. Balachandran et al., U.S. Pat. No. 5,874,384, 1999). However, the engineering critical current density is not high due to the low packing ratio of the superconductor. Progress has been made in improving the engineering critical current density by increasing the superconductor-silver interface, but this is limited by the formation of interlinks between the superconducting filaments when the filaments and the silver layers between them become thinner and thinner. A method to improve the texture formation inside the filament is a more desirable solution.